BY ENGINE/4a/quadtuning
From AE86 Driving Club FAQ
For people with multiple throttles the intake system length is somewhat easily changed in length via the trumpets/air-box, this opens up the options of tuning the intake to suit the engine and driver. While this can also be done on a single throttle engine it is considerably harder and this guide is written with the focus on tuning multiple throttle engines only, however it can be applied to both.
Basic principles of harmonic resonance in the intake
When the intake valve of an engine opens a low pressure wave travels up the intake until it hits the end of the trumpet/plenum entry and this then reflects a low pressure wave back down the pipe. During which time the engine has gone though its combustion. If the pressure pulse has taken the right amount of time it will hit the intake valve just as it is opening sending a positive pressure wave into the intake of the engine. This however will only occur at its greatest at one point in the rev range (variable cam timing exempted). If an engine is at the right rev range for this to happen it is having a harmonic resonance in the intake. This means that in theory the power compared to standard will slowly pick up through the rev range till it reaches the tuned point and then diminish. Also note that the tuned rpm point of the intake is not necessarily where the maximum power is, for example the intake can be tuned for 4000RPM but the engine could still make maximum power at 7000RPM.
Adjusting the intake length to suit
To adjust the rev range at which the resonance will happen the length of the intake can be changed, for owners of 4a engines with quad throttles this is relatively easy. Basic theory is that the longer the trumpet the longer it will take the pulse to reach the end and come back, this will work for a lower rev range because the engine at lower revs will take longer to go through its 4 strokes and hence the time delay needs to be longer to get the right resonant frequency. However this is only a rough rule as despite the theory sometimes engines act in a way that is unexpected and 4age engines are a good example of this. One report is that 100mm length trumpets on a 20V engine gave overall good power and the best top end despite the shorter lengths theoretically optimising the top end power better.
One possible option is to have an easily altered setup using on the end of the throttle bodies is the flange with a short section of pipe welded on it, a set of trumpets with no flange at all and a selection of rubber hoses. By bolting the flange on, clamping the rubber hose to the short section and then clamping it to the trumpet it is possible to change the length by chopping down the rubber pipe. While highly un-recommended for anything more than basic testing it is ideal for tuning work on a dyno or on the move on the road.
Determining the theoretical ideal length
It is possible to work out the length at which the intake will theoretically resonate at, to work this out the angle at which the intake valve closes and the length of the inlet tract is needed. An excellent guide on working this out is available at the website of “Mr acoustic”, it is located here:
Matching an intake to a 20V engine
However as mentioned previously the ideal theoretical length may be quite different than the real world ideal length.
Tuning with variable valve timing (VVT)
The 4age 20V is fitted with a very simple form of variable valve timing that allows for two different inlet cam timing positions depending on the RPM. This alters when the valve closes and therefore there is two points in the rev range where the intake resonates.
Integral adjustable lengths
Originally pioneered by formula one technicians, a variable length set of intakes that worked in proportion to revs were fitted to their engines. This system worked by having the trumpets slide into the runner to change the effective length. Car manufactures over time also incorporated variable systems that were/are primitive compared to the formula one technology but was still quite effective. Most use/used butterflies to select a shorter or longer intake path for the air to travel down. Although there are no known reports, it is theoretically possible for a home builder to make a custom manifold to incorporate such a system. For this a system could be adapted off an engine that has it fitted as original equipment. A rev based output on an EMS or a simple rev based switch could be used to easily control the changeover point.
Harmonic resonance tuning and forced induction
It is believe by many engine tuners that have experimented with forced induction engines (supercharged/turbocharged) that the effect of intake tuning is for the greater part negligible when the engine is on boost. This does however open up the possibility of using a longer than normal intake length to help with off-boost response and possibly even assist in bringing the engine onto boost sooner.
Air-box (if used)
There are many different opinions on what is an ideal shape and volume for an air-box. However as a rough rule the wall of the air-box should not be any closer to the trumpet opening that the equivalent length of the trumpet diameter. For example if a trumpet has a 50mm diameter then from the bell-mouth opening should be at least 50mm from the edge of the air-box wall. In regard to the volume many tuners quote to have equal to at least the same volume as the engine displacement (ex: 1.6L air-box for a 1.6L engine). However unlike the plenum in a single throttle engine increasing the volume of the air-box does not directly increase “throttle-lag”, so it might well be worth using favouring a larger volume. On a separate matter when designing the air-box allow for a good air inlet that will let the air smoothly flow into the chamber and be able to evenly reach to all the cylinders.
Pictures by Sam_Q [[1]]
Picture courtesy of Chris (Takai) [[2]]
Filtration
A massive advantage of running a fully enclosed air-box is that a very large low restriction filter can be placed remotely for all the cylinders. Using this as an advantage the filter itself can be placed in an area near the front of the car where cold air will flow over or a duct be run to the filter to supply cold air.
For many people the choice is to have no air-box at all and have some form of filtration directly over the trumpets (or none at all!) combined with a MAP/TPS based load sensing for the engine management. However caution needs to be taken in this area as the oil in these filters can be quite flammable and they can be amazingly restrictive depending on the model. Tests need to be done with and without the filters to see what effect they have on the airflow.
A cold air intake on a 4age 20v powered AE86 with a original air-flow meter and a K&N branded paper filter.
Picture by Sam_Q [[3]]
A 4age 20V engine with aluminium trumpets covered with foam 'Unifilter' branded filters.
Picture by Sam_Q [[4]]
The result of a backfire igniting the air filter oil of some foam filters
Picture courtesy of Chris (Takai) [[5]]
Trumpets
It must be noted even if a custom air-box is made with long runners there has to be some form of flared edge, inward taper or bell-mouth. Without this the air produces significant turbulence gaining entry to each runner and on a flow-bench this will have a 30% loss in maximum airflow for the pipe. As for the shape of the end of the trumpet itself the results vary. It in experiment conducted on a flow-bench it tested the following: a pipe with a bell-mouth that the lip curled around to a full 180 degrees, a pipe with a flared end at 45 degrees and lastly a straight section of pipe. Below are the results using the bell-mouthed end as a starting point:
Bell-mouthed end: 100% flow
Flared end: -3% flow
Straight pipe: -30%
As shown a full bell-mouth is only a marginal improvement over a basic flared end, for those on a budget any car exhaust shop can form such a shape out of a piece of exhaust pipe. A bell-mouthed end can also be made at home with a very low cost although it does require a high degree of welding ability. It Is possible to use the centre out of an stainless egg holder welded to a two inch steel pipe. See pictures below for further reference.
A picture of flared end trumpet made from exhaust pipe and finished with a die grinder
STEP 1: A humble egg holder upside down purchased for $2 at a supermarket
STEP 2: The bottom chopped off and a section of two inch exhuast pipe welded on
STEP 3: The inside cleaned up
All above pictures by Sam_Q [[6]]
Packaging
Unfortunately even when an intake tuning length is found to be ideal it sometimes may have to be abandoned for a more inferior size because of packaging reasons. For any right hand drive vehicle with the intake on the same side the air-box and trumpets are all fighting for space with the steering system, brake master cylinder & booster, clutch master cylinder, etc… For pure race setups with inclined intake ports it can be relatively easy as the whole intake system can be placed in a continuous line so as to face upwards missing everything. For others the most practical way to have a reasonable length intake is to have curved trumpets. Although shunned by many having curves in an intake system isn’t necessarily a bad thing providing mandrel bends are used throughout. In many cars a good option is to face the trumpets downwards into an air-box that has only bell-mouths on the pipe inlets instead of fully formed trumpets. Under no circumstances should a person from lack of space have a trumpet go to a closer measurement than that of the width of the inlet diameter. For example some people because of lack of space in 4age powered AE86’s run their fourth trumpet quite close to the brake booster, this has the potential for uneven air-flow between that cylinder and the rest. Also trumpets shouldn’t be staggered in length, more on this point in the next section.
Unequal lengths in the Intake
For packaging reasons or otherwise some people have fitted different length trumpets and this has the potential for significant effects on the engine. In theory it’s good, instead of a single point in the rev range that all the pipes work in resonance, it is instead spread over multiple points in the rev range (ex: for a more linear power curve). The shorter pipes will theoretically resonate at the higher revs and the longer ones at lower revs. However in practice this isn’t so simple. Modern original equipment and aftermarket engine management systems (EMS) all work by taking signals from the sensors or airflow meters (if used) to determine the load on the engine as a whole and then determine the fuel and spark required from a pre-programmed “maps”. No normal engine management system can determine individual airflows per cylinder, it is assumed that the load and airflow for each cylinder is identical and that’s where the problem lies. What this means is that if one of the cylinders is sucking a disproportionate amount of air then the will not be able to compensate and therefore causing an incorrect mixture and spark advance for that cylinder.
For example if we have a theoretical engine setup that has two long and two short trumpets. If the longer trumpets starts to resonate during the mid range and the rest are not because they are shorter, then it means that the longer trumpets are getting more air in them that the other two cylinders. This will mean that any point around those tuned revs the longer trumpets will run lean and the shorter trumpets will run more rich. At higher revs the shorter trumpets will resonate and therefore the whole situation will be reversed. Greater caution needs to be taken as the revs increase as the possibilty of engine damage increases significantly.
Article and all pictures by Sam_Q [[7]]
Please let me know if you have any suggestions or additional information/pictures that may be of use.
